The hydraulic system of a machine, such as an excavator or a loader, typically includes a pump, a valve, and a hydraulic actuator in fluid communication. The hydraulic actuator may be a hydraulic cylinder, a hydraulic motor, or another device supplying motive power to a work implement or drive train of the machine. When an operator of the machine actuates a valve by, for example, moving a lever, pressurized hydraulic fluid flows from the pump to the hydraulic actuator through the valve to move a work element of the hydraulic actuator, such as a piston in a hydraulic cylinder.
The speed at which the hydraulic actuator moves is proportional to the rate of hydraulic flow through the valve, which in turn is a function of a valve position and pressure differential across the valve. Hydraulic flow to the hydraulic actuator can be controlled by changing the position of a valve spool located in the valve, but consideration must also be given to load pressure in the hydraulic actuator. For example, the hydraulic actuator may undesirably move much more quickly when lifting a light load than a heavy load at a single valve position, due to low pressure on the hydraulic actuator side of the valve. Traditionally, operators have compensated for this effect by reducing the engine speed and consequently the pump speed, thereby lowering pressurized fluid supply from the pump and the resultant pressure differential across the valve.
A variable displacement pump is often used in a machine. The variable displacement pump generally includes a drive shaft, a rotatable cylinder barrel having multiple piston bores, and pistons held against a tiltable swashplate biased by a centering spring. When the swashplate is tilted relative to the longitudinal axis of the drive shaft, the pistons reciprocate within the piston bores to produce a pumping action and discharge the pressurized hydraulic fluid.
It is well known in the hydraulic field to automatically control the swashplate angle of the pump such that the flow to the hydraulic actuator is maintained at a predetermined pressure, slightly higher than the maximum load pressure required by the system. This type of system is typically referred to as a pressure compensated pump.
By utilizing the output pressure to control the pump displacement, the variable displacement pump can reduce its pump displacement to a minimum level when no flow is required to move the actuator, i.e., when the valve is closed. In such a pump, reduction in pressure when the valve is opened causes the pump to increase the pump displacement to maintain the constant output pressure. However, under light load conditions the predetermined output pressure often results in a pressure differential across the valve causing device movements or acceleration faster than what the machine operator desires. Furthermore, reduction of the engine speed will not reduce the pump output, due to the automatic adjustment of the pump displacement to maintain the predetermined output pressure.
It is also known to provide load compensation by maintaining a predetermined differential pressure across the valve at any load pressure and consequently controlling the flow rate and actuator speed regardless of changing load conditions. In U.S. Pat. No. 5,447,093, a flow force compensation system is disclosed for maintaining a predetermined pressure differential across a directional valve. The system provides flow force compensation by a forced balancing device connected to a pressure compensated valve. The flow force compensation system may not, however, offer a desired flexibility in actuator response for different operations. Machine operators may prefer to have different response based on the operation performed. For example, an operator may prefer more rapid movement at a given control lever position when digging a trench than when craning a light object.
Thus, it is desirable to provide a hydraulic flow control system that provides flexible control of actuator response based on operational conditions. The present invention is directed to solving one or more of the problems associated with prior art designs.